The present invention relates generally to systems for monitoring the flow rate of recirculated exhaust gas in an exhaust gas recirculation (EGR) system of an internal combustion engine, and more specifically to systems for diagnosing faults or failure conditions associated with EGR flow rate operation.
When combustion occurs in an environment with excess oxygen, peak combustion temperatures increase which leads to the formation of unwanted emissions, such as oxides of nitrogen (NOx). This problem is aggravated through the use of turbocharger machinery operable to increase the mass of fresh air flow, and hence increase the concentrations of oxygen and nitrogen present in the combustion chamber when temperatures are high during or after the combustion event.
One known technique for reducing unwanted emissions such as NOx involves introducing chemically inert gases into the fresh air flow stream for subsequent combustion. By thusly reducing the oxygen concentration of the resulting charge to be combusted, the fuel burns slower and peak combustion temperatures are accordingly reduced, thereby lowering the production of NOx. In an internal combustion engine environment, such chemically inert gases are readily abundant in the form of exhaust gases, and one known method for achieving the foregoing result is through the use of a so-called Exhaust Gas Recirculation (EGR) system operable to selectively introduce (i.e., recirculate) exhaust gas from the exhaust manifold into the fresh air stream flowing to the intake manifold via a controllable EGR valve. Through the use of an on-board microprocessor, control of the EGR valve is typically accomplished as a function of information supplied by a number of engine operational sensors.
It is desirable to monitor EGR flow rate operation to insure that the actual EGR fraction tracks the desired or commanded EGR fraction with a specified accuracy. What is therefore needed is a system for monitoring EGR flow rate operation and diagnosing EGR flow rate faults or failure conditions as they may occur.
The present invention may comprise one or more of the following features and combinations thereof. A system for diagnosing exhaust gas recirculation (EGR) flow rate operation in an internal combustion engine may comprise intake and exhaust manifolds each operatively coupled to the engine, and an EGR conduit for recirculating exhaust gas from the exhaust manifold to the intake manifold. Means are provided for determining an EGR fraction corresponding to a fractional amount of recirculated exhaust gas in a total air charge supplied to the intake manifold, and a control computer is configured to continually accumulate at least one EGR fraction error sum as a function of the EGR fraction and a desired EGR fraction, and to diagnose EGR flow rate operation as a function of the accumulated value of the at least one EGR fraction error sum.
The control computer may be configured to diagnose the EGR flow rate operation as failing if the accumulated value of the at least one EGR fraction error sum meets or exceeds one of a first and a second threshold value during a diagnostic period. The control computer may be configured to set the EGR flow rate diagnostic flag to FAIL upon diagnosing the EGR flow rate operation as failing.
The control computer may further be configured to diagnose the EGR flow rate operation as normal if the accumulated value of the at least one EGR fraction error sum remains less than the one of the first and second threshold value during the diagnostic period and is less than one of a third and fourth threshold value upon expiration of the diagnostic period. The system may further include a memory having an EGR flow rate diagnostic flag stored therein, wherein the control computer is configured to set the EGR flow rate diagnostic flag to PASS upon diagnosing the EGR flow rate operation as normal.
The control computer may further be configured to set the EGR flow rate diagnostic flag to ABORT if the accumulated value of the at least one EGR fraction error sum remains less than the one of the first and second threshold value during the diagnostic period but meets or exceeds the one of a third and fourth threshold value upon expiration of the diagnostic period.
The control computer may be configured to continually compute for the diagnostic period an EGR fraction error as a difference between the EGR fraction and the desired EGR fraction, a positive EGR fraction error sum as a maximum of zero and a sum of a previous value of the positive EGR fraction error sum and the EGR fraction error, and a negative EGR fraction error sum as a maximum of zero and a sum of a previous value of the negative EGR fraction error sum and the EGR fraction error.
In this embodiment, the control computer may be configured to diagnose the EGR flow rate operation as failing if either of the accumulated value of the positive EGR fraction error sum meets or exceeds the first threshold value during the diagnostic period and the accumulated value of the negative EGR fraction error sum meets or exceeds the second threshold value during the diagnostic period.
The control computer may further be configured in this embodiment to diagnose the EGR flow rate operation as normal if the accumulated value of the positive EGR fraction error sum remains less than the first threshold value throughout the diagnostic period and the accumulated value of the negative EGR fraction error sum remains less than the second threshold value throughout the diagnostic period, and if the positive EGR fraction error sum is less than a third threshold value, less than the first threshold value, and the negative EGR fraction error sum is less than a fourth threshold value, less than the second threshold value, upon expiration of the diagnostic period.
The control computer may further be configured in this embodiment to set the EGR flow rate diagnostic flag to ABORT if the accumulated value of the positive EGR fraction error sum remains less than the first threshold value throughout the diagnostic period and the accumulated value of the negative EGR fraction error sum remains less than the second threshold value throughout the diagnostic period, and if the positive EGR fraction error sum meets or exceeds the third threshold value and the negative EGR fraction error sum meets or exceeds the fourth threshold value, upon expiration of the diagnostic period.
The control computer may further be configured to monitor a number of diagnostic enable conditions prior to starting the diagnostic period, and to start the diagnostic period only if all of the number of diagnostic enable conditions are met. The control computer may further be configured to monitor the number of diagnostic enable conditions throughout the diagnostic period, and to restart the diagnostic period if any of the number of diagnostic enable conditions is no longer met.
The system may include means for determining rotational speed of the engine, and one of the number of diagnostic enable conditions may correspond to the rotational speed of the engine being within a predefined range of engine rotational speeds.
The control computer may further be configured to determine an engine load value as a function of an engine fueling command, and one of the number of diagnostic enable conditions may correspond to the engine load value being less than a threshold engine load value.
The system may further include an EGR valve disposed in-line with the EGR conduit, wherein the EGR valve is responsive to an EGR valve control command to control a position of the EGR valve relative to a reference position, and means for determining the position of the EGR valve relative to the reference position. One of the number of diagnostic enable conditions may correspond to the position of the EGR valve being greater than a threshold EGR valve position.
The system may further be configured to monitor a number of data acquisition conditions prior to during the diagnostic period, and to compute the EGR fraction error, the positive EGR fraction error sum, and the negative EGR fraction error sum only if all of the number of data acquisition conditions are met.
These and other objects of the present invention will become more apparent from the following description of the illustrative embodiments.